1. Field of the Invention
The present invention relates to a method and a device for balancing the charges of a plurality of series-connected battery cells.
2. Description of the Prior Art
The devices known for balancing the charges of series connected battery cells are based either on a comparison of the individual voltage of each battery cell with an overcharge voltage, either upon actuation of an overcurrent relay, connecting the different battery cells together. These techniques are not adapted for lithium battery cells because current or voltage overcharge can cause the destruction of battery cells.
EP-0 767 524 (MOTOROLA) describes a method for balancing the charge distributed among two series connected cells. This method consists in comparing the currents through each of the cells and discharging the cell whose charging voltage is the highest when the difference between the detected currents is greater then a predetermined value. One drawback of this device that, on the one hand, currents can reach high values and damage the cells, and on the other hand, that the arrangement required for carrying out this comparison is complex.
It is an object of the present invention to provide a method and device for balancing the charges of series-connected battery cells while avoiding the destruction of such battery cells due to a voltage or current overcharge.
This object is achieved by a method wherein a current dissipative loop is connected across each battery cell, the voltage across each battery cell is measured, an average value of the measured voltages is determined, the measured voltage is compared to said average value, and for a given battery cell, the measured voltage is adjusted to said average value so as to equalize charges of all battery cells.
For this purpose, the dissipative loop is closed if the voltage measured across the relevant battery cell is greater than the average of the measured voltages, and said dissipative loop is opened if the voltage measured across said battery cell is lower than the average of the measured voltages.
The method according to this invention is carried out by a device comprising means for measuring the individual voltage of each battery cell, means for determining the average voltage of all series-connected battery cells and control means for equalizing the measured voltage with said determined average voltage.